The present invention relates to a chlorine-containing resin composition containing Mg—Al-based hydrotalcite-type particles, the Mg—Al-based hydrotalcite-type particles, and a process for producing the Mg—Al-based hydrotalcite-type particles. More particularly, the present invention relates to a chlorine-containing resin composition containing Mg—Al-based hydrotalcite-type particles having a large plate surface diameter and an adequate thickness, which is not only excellent in heat stability and heat discoloration resistance but also is improved in electrical insulating property; the Mg—Al-based hydrotalcite-type particles which are suitable as a stabilizer for chlorine-containing resins; and a process for producing such Mg—Al-based hydrotalcite-type particles.
As layered compounds, there are known various compounds as well as clay minerals or the like. Among these layered compounds, layered double hydroxides such as hydrotalcite have such a structure capable of inserting various anions or molecules into spacings between respective layers thereof and, therefore, can exhibit an anion-exchanging property.
In general, as described in Journal of the Chemical Society of Japan, 1995, No. 8, pp. 622 to 628, the hydrotalcite is represented by the formula:[M2+1−xM3+x(OH)2]x+[An−x/n.yH2O]x−wherein M2+ is a divalent metal ion such as Mg2+, Co2+, Ni2+ or Zn2+; M3+ is a trivalent metal ion such as Al3+, Fe3+ or Cr3+; An− is an n-valent anion such as OH−, Cl−, CO32− or SO42−; and x is usually 0.2 to 0.33, and such hydrotalcite has a laminated crystal structure which comprises two-dimensional main layers composed of regular octahedral brucite units each having a positive charge, and interlayers each having a negative charge.
The hydrotalcite has been used in various applications because of good anion-exchanging property thereof, for example, as ion exchange materials, adsorbents, deodorants or the like. Also, the hydrotalcite has been used in various other applications such as stabilizers for resins or rubbers, e.g., polyethylene, polypropylene and chlorine-containing resins (typically, vinyl chloride resins), as well as paints, various catalysts, agricultural films, inks or the like.
In recent years, as to catalysts or the like, it has been required to use those containing no harmful metals from the standpoint of environmental protection. The hydrotalcite-type particles have been expected to satisfy these requirements because the hydrotalcite-type particles have almost no toxicity and exhibit an excellent catalytic property or the like.
In particular, among these hydrotalcite-type particles, Mg—Al-based hydrotalcite-type particles containing Mg2+ as a divalent metal ion and Al3+ as a trivalent metal ion, are most noticeable owing to its good stability.
As the general production method of hydrotalcite, there is known a method of mixing an aqueous metal salt solution containing divalent metal ions and trivalent metal ions which constitute main layers thereof, with an aqueous carbonate solution containing carbonate ions which constitute interlayers thereof, and then subjecting the obtained mixture to coprecipitation reaction while controlling the temperature, the pH value and the like. In addition to the above method of conducting the reaction under ordinary pressure, there is also known a method of producing hydrotalcite under pressure by hydrothermal reaction using an autoclave.
Hitherto, hydrotalcite-type particles used for kneading into resins, have been required to possess a large plate surface diameter and an adequate thickness in the consideration of dispersibility in resins upon kneading. However, in order to obtain such hydrotalcite-type particles having a large plate surface diameter, it is necessary to use specific reaction conditions such as those for hydrothermal synthesis or the like.
Further, in the case where the hydrotalcite-type particles are used as a stabilizer for chlorine-containing resins or rubbers such as vinyl chloride resins or the like, it is also required that the particles are sufficiently dispersed in the resins or rubbers and have a high chlorine ion-capturing ability in order to obtain resin or rubber products having an excellent heat resistance.
More particularity, among the above-described various applications, the use of hydrotalcite-type particles as a stabilizer for chlorine-containing resins is more noticeable. Such chlorine-containing resins have been used in various applications because of good physical and chemical properties thereof. Especially, the chlorine-containing resins are more excellent in electrical insulating property, arc resistance, tracking resistance and voltage resistance as compared to polyolefins and, therefore, have been widely used as an electric wire-covering material.
However, the chlorine-containing resins are susceptible to heat deterioration and oxidation degradation and, therefore, suffer from deteriorated properties such as low elongation when used for a long period of time, thereby causing such a problem that the resins are no longer usable as an electric wire-covering material.
For this reason, as the stabilizer for chlorine-containing resins as an electric wire-covering material, there have been frequently used lead-based compounds such as tribasic lead sulfate, lead stearate or the like. However, it has been recently required to use those materials containing no toxic metal from the standpoint of environmental protection. Therefore, the use of hydrotalcite-type particles having substantially no toxicity and an excellent function as a stabilizer for chlorine-containing resins, has been proposed (Japanese Patent Application Laid-Open (KOKAI) Nos. 55-80445, 57-80444, 57-147552, 58-122951 and 8-73687 (1996)).
More specifically, in Japanese Patent Application Laid-Open (KOKAI) No. 8-73687(1996), it has been described that a halogen-containing resin composition comprising a halogen-containing resin, specific antioxdants, hydrotalcites and zinc compounds. The hydrotalcites used in Japanese Patent Application Laid-Open (KOKAI) No. 8-73687(1996), is represented by the formula:Mgx.Al2(OH)2x+4CO3.nH2O
wherein 4≦x≦6, 0≦n≦20. Further, as the hydrotalcites used in Examples of Japanese Patent Application Laid-Open (KOKAI) No. 8-73687(1996), there is cited Mg4.5.Al2(OH)13CO3.3.5H2O and Mg4.Al2(OH)12CO3.3H2O.
The chlorine-containing resin compositions used as an electric wire-covering material have been required to possess an excellent heat stability and a high electrical insulating property. Further, those resin compositions used as white- or light-colored electric wire-covering materials have been required to be free from heat discoloration upon processing, i.e., to show a good heat discoloration resistance. Further, in order to obtain chlorine-containing resin compositions capable of satisfying the above requirements, it is necessary that hydrotalcite-type particles used as a stabilizer have not only a large plate surface diameter and an adequate thickness but also a high chlorine ion-capturing ability.
At the present time, it has been strongly demanded to provide Mg—Al-based hydrotalcite-type particles which have a large plate surface diameter and an adequate thickness, and is suitable as a stabilizer for chlorine-containing resins. However, such Mg—Al-based hydrotalcite-type particles capable of satisfying the above properties have not been obtained yet.
In the conventional coprecipitation methods, it is not possible to obtain hydrotalcite-type particles having a large plate surface diameter. Under specific reaction conditions such as those of hydrothermal synthesis, although hydrotalcite-type particles having a large plate surface diameter can be produced, when such hydrotalcite-type particles are used as a stabilizer of chlorine-containing resins, a sufficient heat resistance can not be obtained.
Especially, it has been strongly demanded to provide a chlorine-containing resin composition which is excellent in heat stability and heat discoloration resistance and is improved in electrical insulating property. However, such a chlorine-containing resin composition capable of satisfying these requirements have not been obtained yet.
Thus, such conventional techniques described in the above prior publications, have failed to provide a sufficient stabilizing effect and, therefore, the heat stability, heat discoloration resistance and electrical insulating property of the conventional chlorine-containing resin compositions are insufficient.
As a result of the present inventors' earnest studies, it has been found that when by kneading in a chlorine-containing resin Mg—Al-based hydrotalcite-type particles having a plate surface diameter of 0.1 to 1.0 μm and a thickness of 0.02 to 0.08 μm, obtained by mixing an aqueous anion-containing alkali solution, an aqueous magnesium salt solution and an aqueous aluminum salt solution with each other, adding an aqueous calcium salt solution to the mixed solution such that the molar ratio of Ca to a sum of Mg and Al is 0.01:1 to 0.2:1, and aging the obtained solution at a temperature of 60 to 105° C. while controlling the pH value of the solution to 10 to 14, the obtained resin composition is considerably improved in heat stability, heat discoloration resistance and electrical insulating property. The present invention has been attained on the basis of this finding.